Apparatus and method for measuring polarization dependent loss

Abstract

An apparatus for measuring polarization dependent loss is disclosed featuring several fiber optic couplers combined in tandem and oriented such that the PDL noise of the measurement system is reduced to a negligible level. By matching the PDLs of the couplers and vectorally subtracting opposite phases of polarization, the PDL of the measurement system is virtually eliminated. Thus, the PDL noise floor is lowered to near zero and the PDL of the optical device-under-test (DUT) can be accurately measured. The system is relatively inexpensive to implement and offers needed versatility because it measures the PDL of optical devices that operate in a reflection mode or in a forward transmission mode. Thus, it provides one PDL measurement solution for both types of devices.

What is claimed is:  
     
       1. An apparatus for measuring a polarization dependent loss of an optical device under test using a randomly polarized light signal, said apparatus generating a test input signal that is directed into said optical device under test, said apparatus comprising: 
       a first passive optical element connected to said light source, wherein said first passive optical element has a first polarization dependent loss;  
       a second passive optical element connected to said first passive optical element and having a second polarization dependent loss that is substantially equal to said first polarization dependent loss, said second passive optical element being disposed relative to said first passive optical element such that the second polarization dependent loss substantially cancels the first polarization dependent loss and outputs the test input signal with a polarization dependent loss equal to a first minimum value;  
       a measurement element connected to the output of the optical device under test for measuring the device output signal and calculating the optical device under test polarization dependent loss; and  
       a calibration assembly connected to the second passive optical component for calibrating a measurement in a forward transmission mode, said calibration assembly rotates the second passive optical component in response to a calibration signal to thereby cause the second polarization dependent loss to substantially cancel the first polarization dependent loss.  
     
     
       2. The apparatus according to  claim 1 , wherein the measurement element further comprises: 
       a light detector connected to the device under test; and  
       a computing device connected to said light detector, said computing device includes a software program having a measurement routine and a calibration routine.  
     
     
       3. An apparatus for measuring a polarization dependent loss of an optical device under test using a randomly polarized light signal, said apparatus generating a test input signal that is directed into said optical device under test, said apparatus comprising: 
       a first passive optical element connected to said light source, wherein said first passive optical element has a first polarization dependent loss; and  
       a second passive optical element connected to said first passive optical element and having a second polarization dependent loss that is substantially equal to said first polarization dependent loss, said second passive optical element being disposed relative to said first passive optical element such that the second polarization dependent loss substantially cancels the first polarization dependent loss and outputs the test input signal with a polarization dependent loss equal to a first minimum value;  
       wherein the first minimum value equals the absolute value of the first polarization dependent loss minus the second polarization dependent loss.  
     
     
       4. The apparatus according to  claim 3 , wherein the first minimum value is less than 0.001 dB. 
     
     
       5. An apparatus for measuring a polarization dependent loss of an optical device under test using a randomly polarized light signal, said apparatus generating a test input signal that is directed into said optical device under test, said apparatus comprising: 
       a first passive optical element connected to said light source, wherein said first passive optical element has a first polarization dependent loss; and  
       a second passive optical element connected to said first passive optical element and having a second polarization dependent loss that is substantially equal to said first polarization dependent loss, said second passive optical element being disposed relative to said first passive optical element such that the second polarization dependent loss substantially cancels the first polarization dependent loss and outputs the test input signal with a polarization dependent loss equal to a first minimum value;  
       wherein the test input signal is reflected by the optical device into the second passive optical component to thereby generate a test output signal that is directed into an output of the second passive optical component.  
     
     
       6. The apparatus according to  claim 5 , further comprising: 
       a third passive optical component connected to the output of the second passive optical component and having a third polarization dependent loss substantially equal to said second polarization loss, said third passive optical component being disposed relative to the second passive optical component such that the third polarization dependent loss cancels the second polarization dependent loss to thereby cause the test output signal to have a polarization dependent loss substantially equal to a second minimum value.  
     
     
       7. The apparatus according to  claim 5 , wherein the second minimum value equals the absolute value of the second polarization dependent loss minus the third polarization dependent loss. 
     
     
       8. The apparatus according to  claim 7 , wherein the second minimum value is less than 0.001 dB. 
     
     
       9. The apparatus according to  claim 6 , wherein the first passive optical component, the second passive optical component, and the third passive optical component are couplers. 
     
     
       10. The apparatus according to  claim 6 , further comprising: 
       a measurement element connected to the third passive optical component for measuring the test output signal and calculating an optical device under test polarization dependent loss; and  
       a calibration assembly connected to the second passive optical component and the third passive optical device for calibrating a measurement in a forward transmission mode and a backward reflection mode.  
     
     
       11. The apparatus according to  claim 10 , wherein the measurement device further comprises: 
       a light detector connected to the device under test; and  
       a computing device connected to said light detector, said computing device includes a software program having a measurement routine and a calibration routine.  
     
     
       12. The apparatus according to  claim 11 , wherein the measurement device calculates a test output signal polarization dependent loss by solving:        PDL   =         Log   10          [       P     out                 max         P   in       ]       -       Log   10          [       P     out                 min         P   in       ]                         
       wherein PDL is said test output signal polarization dependent loss in decibels, P out max  is the maximum power of the test output signal, P out min  is the minimum power of the test output signal, and P in  is the power of the test input signal. 
     
     
       13. The apparatus according to  claim 12 , wherein the optical device under test polarization dependent loss equals the test output signal polarization dependent loss minus the second minimum value. 
     
     
       14. The apparatus according to  claim 10 , wherein the calibration assembly further comprises: 
       a rotatable stage assembly for rotating the second passive optical component in the forward transmission calibration mode and the third passive optical component in the backward reflection calibration mode; and  
       a motor assembly for actuating said rotatable stage assembly in response to a signal from the measurement element, wherein said rotatable stage, said motor assembly, and the measurement element form a control loop during the forward transmission calibration mode and the backward reflection calibration mode.  
     
     
       15. The apparatus according to  claim 14 , wherein the measurement element drives the motor to cause the second passive optical component to rotate with respect to said first passive optical element until the polarization dependent loss of the calibration signal is substantially equal to the first minimum value. 
     
     
       16. The apparatus according to  claim 15 , wherein the first minimum value equals the absolute value of the first polarization dependent loss minus the second polarization dependent loss. 
     
     
       17. The apparatus according to  claim 14 , wherein the measurement element drives the motor causing the third passive optical component to rotate with respect to said second passive optical element until the polarization dependent loss of the calibration signal is substantially equal to the second minimum value. 
     
     
       18. The apparatus according to  claim 14 , wherein the measurement element is optically connected to the second passive optical component in the forward transmission calibration mode and optically connected to the third passive optical component in the backward reflection calibration mode to thereby measure a calibration signal power and generate an error signal based on said calibration signal power, wherein the measurement element drives the motor assembly until said error signal equals a first minimum value in the forward transmission calibration mode and a second minimum value in the backward reflection calibration mode. 
     
     
       19. A method for measuring a polarization dependent loss of an optical device, in an apparatus including a light source for emitting a light signal that is randomly polarized, a first passive optical element connected to said light source, wherein said first passive optical element has a first polarization dependent loss, said method for measuring comprising the steps of: 
       providing a second passive optical element having a second polarization dependent loss that is substantially equal to the first polarization dependent loss and connected to the first passive optical element at a relative position such that said second polarization dependent loss substantially cancels the first polarization dependent loss;  
       directing the light signal into the first passive optical component such that said second passive optical component produces a test input signal, wherein said test input signal has a polarization dependent loss substantially equal to a first minimum value;  
       directing said test input signal into the optical device; and  
       measuring an output signal of the optical device to thereby determine the polarization dependent loss.  
     
     
       20. The method according to  claim 19 , wherein the first minimum value equals the absolute value of the first polarization dependent loss minus the second polarization dependent loss. 
     
     
       21. The method according to  claim 19 , further comprising the steps of: 
       providing a third passive optical element having a third polarization dependent loss that is substantially equal to the second polarization dependent loss and connected to the second passive optical element at a rotated position such that said third polarization dependent loss substantially cancels the second polarization dependent loss;  
       directing a test output signal into the second passive optical component, wherein said test output signal is generated by reflecting the test input signal off of the optical device under test;  
       measuring said test output signal at an output of said third passive optical component to thereby generate test output measurements; and  
       calculating the polarization dependent loss using said test output measurements.  
     
     
       22. A method for calibrating an apparatus used for measuring a polarization dependent loss of an optical device, said apparatus including a light source for emitting a light signal that is randomly polarized and a first passive optical element having a first polarization dependent loss, said method for calibrating comprising the steps of: 
       providing a second passive optical element having a second polarization dependent loss that is substantially equal to the first polarization dependent loss and connected to the first passive optical element;  
       directing the light signal into the first passive optical component to thereby create a calibration signal that exits said, second passive optical component; and  
       rotating said second passive optical component until said second polarization dependent loss substantially cancels the first polarization dependent loss and a first calibration polarization dependent loss of said first calibration signal equals a first minimum value.  
     
     
       23. The method according to  claim 22 , wherein the first minimum value equals the absolute value of the first polarization dependent loss minus the second polarization dependent loss. 
     
     
       24. The method according to  claim 22 , further comprising the steps of: 
       providing a third passive optical element having a third polarization dependent loss that is substantially equal to the second polarization dependent loss and connected to the second passive optical element;  
       directing the light signal into the second passive optical component such that a second calibration signal exits said third passive optical component; and  
       rotating said third passive optical component until said third polarization dependent loss substantially cancels the second polarization dependent loss and a second calibration polarization dependent loss of said second calibration signal equals a second minimum value.  
     
     
       25. The method according to  claim 24 , wherein the second minimum value equals the absolute value of the second polarization dependent loss minus the third polarization dependent loss.